This application is related to my copending applications Ser. No. 849,589 entitled "FDM/TDM Transmultiplexer" and Ser. No. 849,279 entitled "A Configurable Parallel Arithmetic Structure For Recursive Digital Filtering," both filed Nov. 7, 1977 and assigned to the same assignee as the present invention.
In each of my above-mentioned applications, I have disclosed a transmultiplexer capable of converting 60-channel super groups from FDM-to-TDM and vice versa. In order to process such a large number of channels, the transmultiplexer must be capable of very high operating speeds. An important aspect of my novel transmultiplexer is that it is relatively small, but it was also my purpose in designing the transmultiplexer to decrease the cost of manufacture and simplify the maintenance of the system and, therefore, a modular design concept was adopted. The modular design would decrease cost by allowing mass production of a limited number of components, and field maintenance would be simplified by enabling the user to merely replace malfunctioning modules and return them to the manufacture of repair. In order to reduce the size of the transmultiplexer hardware, the computational advantage of Fast Fourier Transform (FFT) algorithm and, therefore, FFT processors were required for both FDM-to-TDM and TDM-to-FDM conversion which were capable of operating at high enough speeds to accomodate a 60-channel super group. Up to this time, FFT processors have been too slow for satisfactory transmultiplexing of a large number of channels. Known FFT processors have also been too large, too expensive and consume too much power to meet the size and cost saving goals of my transmultiplexer design.
Since the inputs to the FFT processor in the FDM-to-TDM conversion are all real, it is possible to reduce the size requirements of that FFT processor by operating it in what is commonly known as a "two channel trick" mode. Since a modular design concept for the transmultiplexer was adopted, it was, therefore, preferable to utilize an FFT processor which could easily be switched to the two channel trick mode.
A further requirement of the modular design was that the user be able to treat the processor as a black-box component so that a malfunctioning processor could merely be removed and replaced by a spare. Presently available FFT processors are integral parts of larger systems and, therefore, are incapable of such "stand-alone" operation.
The requirements of the transmultiplexer are also important in other FFT applications. A relatively small and inexpensive FFT processor capable of very high speed and stand-alone operation would also be of significant advantage in a wide variety of systems employing spectral analysis.